Electrocoating technique, especially cationic electrocoating technique, has been drastically progressed with the development of a blocked polyisocyanate curing agent (see Japanese Kokoku Publication (examined) 34238/1980). The blocked polyisocyanate is a compound of which a reactive isocyanate group is blocked with a blocking agent (such as a low molecular weight alcohol). This compound is unblocked at an elevated temperature to produce the isocyanate groups which contribute crosslinking reactions.
The blocked polyisocyanate curing agent has drawbacks in heating loss and distortion of coating. These drawbacks are caused by the low molecular weight blocking agents which produce after unblocking.
Japanese Kokoku Publication 24007/1988 discloses the use of a cyclic urea derivative curing agent which produces isocyanate groups by decomposition upon heating. The curing agent, however, has problems similar to the isocyanate curing agent, because it is decomposed to produce a low molecular weight material.